Pesticide Risk Reduction by Management Practices: An

Chapter 20

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Pesticide Risk Reduction by Management Practices: An Environmental Case Study of the Australian Cotton Industry Angus N. Crossan, Michael T. Rose, and Ivan R. Kennedy Faculty of Agriculture, Food and Natural Resources, The University of Sydney, Sydney, New South Wales 2006, Australia

The Australian Cotton Industry provides a valuable case study into rational methods for the selection and use of agrochemicals. The industry suffered heavy criticism and economic losses in the late 1990s because of pesticide drift issues and contamination of produce. Consequently, the industry invested in research on practices of agrochemical use by growers. Significant changes, documented in a 'best management practices' manual, included the introduction of genetically-modified cotton varieties, the education of growers into reducing the use of broad-spectrum sprays, and the promotion of on-farm wetlands as buffers to limit water-borne pesticide residues moving off-farm and reducing their persistence on-farm. This chapter draws together data resulting from these innovations and assesses the reduction of environmental risk they have afforded.

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© 2007 American Chemical Society

In Rational Environmental Management of Agrochemicals; Kennedy, I., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2007.

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Introduction The question of whether or not pesticides will be used depends on the perceived benefits versus the costs. Rather than being banned outright, useful chemicals with detrimental side effects can be replaced, restricted, or their use continued with management systems in place. The pattern of the continuing use of pesticides beneficial for Australian cotton production illustrates these possibilities. For example, the persistent insecticide DDT [50-29-3] was banned in Australia in 1987 and dieldrin [60-57-1] was banned in 1995 (1,2); more recently, chlorfluazuron [71422-67-8] was withdrawn from registration for cotton because of its demonstrated potential to contaminate beef (3). In contrast, the insecticide endosulfan [115-29-7], which was also found to contaminate beef (4), was not banned because of its efficacy in controlling the major cotton pest Helicoverpa. Instead its use has been restricted and conditions placed on its use to prevent contamination of beef. Meanwhile, the use of other registered pesticides continues with management systems in place to prevent off-site movement and environmental contamination. This chapter draws on the available datafromthe Australian cotton industry to test the claims of reduction of environmental risk from the introduction of these practices. A set of hazard and risk assessments are presented to illustrate changes in hazardfromthe use of genetically modified cotton varieties. The data available for analysis are presented within the accepted risk assessment framework ( i

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Figure I. Change in cotton varieties grown in Australia; adoption of GM technology as Bollgard' II, Ingard", Roundup Ready™ (RR) and stacked varieties. (Data from reference 8).

Hazard and Risk Reduction Assessment A study was conducted to assess the environmental risk of glyphosate used for weed control on glyphosate-resistant genetically enhanced cotton varieties in relation to other commonly used herbicides, including diuron, 2,4-D [94-75-7], fluometuron [2164-17-2], pendimethalin [40487-42-1], prometryn [7287-19-6] and trifluralin.



323 Risk assessments for a series of herbicide application scenarios were divided into two groups, G M and conventional, for comparison. The GM scenarios involved the cropping of genetically engineered, Roundup Ready® Sicot 189RR cotton, with glyphosate used as the main weed control chemical. Irrigation runoff data for glyphosate and other commonly used herbicides were obtained during field studies and used for a risk analysis in relation to water quality guidelines and the toxicology of two freshwater species, rainbow trout (Oncorhynchus mykiss) and water flea (Daphnia sp.). The project consisted of a modelled risk assessment, which involved a series of theoretical herbicide application programs, and a field study involving herbicide treatment programs for weed control in cotton. The field study was undertaken at Auscott Midkin in the Gwydir Valley (northern New South Wales, Australia) and consisted of four trial fields. All four fields were sown with Roundup Ready® Cotton (Sicot 189RR) to ensure no varietal differences were experienced. Two fields were subjected to typical conventional herbicide programs over the 2001/02 growing season whilst the other two were subjected to typical glyphosate-dominated herbicide programs. Other herbicides used in the study included 2,4-D, diquat dibromide [85-00-7], diuron, fluometuron, paraquat dichloride [1910-42-5], pendimethalin, prometryn and trifluralin. The herbicide application scenarios assessed were based upon industry-typical application programs over an entire growing season. The four fields, with gradients of 1:1400, consisted of light to medium grey clay soils. The soils were relatively uniform across the fields with clay content between 50-60%. Soil organic carbon levels were 1 ± 0.2%, and pH rangedfrom7.5 to 8.6. The results for runoff and suspended sediment (>45 pm) mirrored the dissipation rates observed in the soil samples. Concentrations of herbicides in runoff generally decreased during the season because of the decreasing field source. However, the total concentrations in runoff were affected by sediment load, which varied during the season. Glyphosate in runoff was only detected in suspended sediments, which were all well below ANZECC/ARMCANZ water quality guidelines (12). Conversely, diuron was observed to exceed the freshwater low reliability trigger value (0.2 pg L" ) in every sample. Fluometuron and prometryn were applied to fields 29 and 84 only (i.e., conventional programs). Both herbicides were found to partition quite strongly into the aqueous phase. The field results are considered to be consistent with the understanding of the environmental fate and transport of herbicides (13, 14, 15). 1

Risk of Exceeding Prescribed Water Quality Guidelines From the runoff observations made during this study, the probability of exceeding the local guideline values was taken as a measure of risk to the aquatic


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environment (Table I). These probabilities were determined from an exponential line equation fitted to thefrequencydistributions of the residues detected in the runoff sampling. The values indicate the likelihood of exceeding the guideline values under normal agricultural practice, as experienced during the trial. No special experimental conditions were imposed during the study; all chemicals were applied by routine operators using typical application equipment.

Table I: Probabilities that pesticide concentrations in runoff water from cotton fields will exceed the environmental guideline values Guideline (MgL-') 0.2

Chemical Diuron (ID*) Trifluralin fl

Fluometuron (MRL ) Prometryn (MRL ) Pendimethalin (MRI/) Glyphosate a

2.6 100 100 50 370

Conventional (P>Guideline) 0.80 0.58

GM (P>Guideline) 1

0.08 1.OX 10 3.0X10 1.9X10

-

0.58

s

-6

10

8.8X10""

" Where no ANZECC/ARMCANZ (72) guideline values were prescribed MRL values were used. b

Refers to the ANZECC/ARMCANZ (72) "Insufficient Data" condition and therefore a value with greater sensitivity is prescribed.

The ANZECC/ARMCANZ water quality guidelines for environmental protection (72) do not list values for all herbicides registered for use in Australia. To enable some comparison between the products used in this study, surrogate 'maximum residue level' (MRL) values were used for fluometuron, prometryn and pendimethalin. Diuron was detected at concentrations greater than 0.2 pg L" in every sample from the GM fields, with a resulting large probability of exceeding the guideline value. However, the guideline concentration (72) for diuron is a 'low reliability trigger value' and is conservative to ensure ecosystem protection because there is insufficient toxicity data available to determine a more realistic value. Glyphosate posed the smallest probability of exceeding the prescribed guideline value (from Table I). This is likely a combined result of its relatively low mobility, reported in the literature and shown by the lack of detection in runoff samples (16) and its low toxicity and subsequent relatively large ecosystem protection guideline value. 1


325 Relative Risk to Aquatic Ecosystems for Field Experiment Scenarios The field trial results were used to determine the probability of exposure and the average and median level of exposure. An expression of actual risk was difficult given the scale of the experiment, however a relative assessment including a factor based upon observations was produced. The probability of exposure was determined using observations above the limit of detection (LOD) in the runoff and suspended sediment samples. Exposure values (X) were determined by X = C x P x t x BCF. Where C is the concentration in runoff, P is the probability that residues will be found in runoff, tv is the half-life, indicating persistence, and BCF refers to the bioconcentration factor. The concentrations of herbicides used for the ecosystem risk assessment detailed in this section are shown in Table II. The average and median data were determinedfromthe data set obtainedfromrunoff in the field trials. The median values were selected to represent exposure values. m


2

Table II: Average and median values of the herbicide concentrations in runoff reported from the field trials

Chemical Glyphosate Diuron Fluometuron Prometryn Pendimethalin Trifluralin*

Conv. Ave. (HXL-') 5.33

GM Ave.

Conv. Median

GM Median (HZU')

Pesticide Risk Reduction by Management Practices: An

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